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Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression

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Teklay, M and Scherer, EE and Mezger, K and Danyushevsky, LV (2010) Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression. Contributions to Mineralogy and Petrology, 159. pp. 731-751.

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Abstract

The Afar Depression offers a rare opportunity
to study the geodynamic evolution of a rift system from
continental rifting to sea floor spreading. This study presents
geochemical data for crustal and mantle xenoliths and
their alkaline host basalts from the region. The basalts have
enriched REE patterns, OIB-like trace element characteristics,
and a limited range in isotopic composition
(87Sr/86Sr = 0.70336–0.70356, eNd = ?6.6 to ?7.0, and
eHf = ?10.0 to ?10.7). In terms of trace elements and
Sr–Nd isotopes, they are similar to basalts from the Hanish
and Zubair islands in the southern Red Sea and are thus
interpreted to be melts from the Afar mantle. The gabbroic
crustal xenoliths vary widely in isotope composition
(87Sr/86Sr = 0.70437–0.70791, eNd = -8.1 to ?2.5, and
eHf = -10.5 to ?4.9), and their trace element characteristics
match those of Neoproterozoic rocks from the
Arabian–Nubian Shield and modern arc rocks, suggesting
that the lower crust beneath the Afar Depression contains
Neoproterozoic mafic igneous rocks. Ultramafic mantle
xenoliths from Assab contain primary assemblages of fresh
ol ? opx ? cpx ? sp ± pl, with no alteration or hydrous
minerals. They equilibrated at 870–1,040C and follow a
steep geothermal gradient consistent with the tectonic
environment of the Afar Depression. The systematic variations
in major and trace elements among the Assab mantle
xenoliths together with their isotopic compositions suggest
that these rocks are not mantle residues but rather series of
layered cumulate sills that crystallized from a relatively
enriched picritic melt related to the Afar plume that was
emplaced before the eruption of the host basalts.

Item Type: Article
Keywords: Afar Depression Xenoliths Cumulates Rifting Sr–Nd–Hf isotopes
Journal or Publication Title: Contributions to Mineralogy and Petrology
Page Range: pp. 731-751
Identification Number - DOI: 10.1007/s00410-009-0451-0
Additional Information:

The final publication is available at http://www.springerlink.com

Date Deposited: 08 Dec 2010 04:51
Last Modified: 18 Nov 2014 04:15
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